Preparation of galvanized sheets to receive resinous coatings



Patented Sept. 17, 1946 PREPARATION OF GALVANIZED SHEETS TO RECEIVERESINOUS COATINGS George R. Hoover and Noble E. Hays, Middletown, Ohio,assignors to The American Rolling Mill Company, Middletown, Ohio, acorporation of Ohio No Drawing. Application July 30, 1940, Serial No.348,442

6 Claims.

Our invention relates to the preparation of ferrous sheets to receiveresinous coatings, usually referred to as baked enamels. Enameled sheetsand structures formed from sheets of this Without desiring to be boundby theory we believe that blistering is caused by moisture and hydrogencoming from the sheet surface and failing to escape through the enamel.Formerly character are today widely used in industry-as 5 most of theenamels which were applied to metal one example, in the making of thecabinets of parts and then heated to a drying temperature, householdrefrigerators. For many uses galvancontained, along with a suitablevehicle, thermoized sheets are desirable; and it has come to be plasticresins which softened at the elevated drythe practice to give suchsheets a pretreatment ing or baking temperature but which increased topromote their acceptance of the enamel and the viscosity of the enamelvehicle sufliciently so the bonding of the enamel to the sheets. thatthe enamel would not run off and produce In the early days of themanufacture of baked too thin a coating. The elevated temperature enamelstructures relatively little diificulty was had to be maintained untilthe vehicle polymerencountered with blistering, though a number of izedand hardened. There was thus a considercomplaints arose. The comparativeunimporta e ak g t during Which the Viscosity was ance of blistering wasdue to the substantially not so high, as to prevent the escape of anybubuniversal use of enamel substances of continubles of entrained gas.In the newer types of ously thermoplastic character. More recently,enamel a thermo-setting resin is used which, however, there has been ashift to enamels which with the vehicle, not only makes the enamel moreconsist of resins of thermo-setting type, or conviscous as applied,thereby yielding a heavier tain large quantities of such resins; andblistercoating, but in itself undergoes a chemical ing difilculties havebecome serious. change so that it hardens and becomes perma- Theprincipal objects of our invention are the nently infusible. This typ oenamel hae the solution of the blistering problem, and specifiadvantageof. setting much more quickly and at cally the provision of a treatmentfor the sheets taming the maximum hardness after a shorter which iseffective to prevent blistering, baking operation. It has thedisadvantages of It is an object of our invention to provide a hamperingthe escape of bubbles of gas forming treatment for the purpose which maybe applied therein, of forming a film which tends to entrap by themanufacturer of the sheets, and which the gases, and of hardening sorapidly as to preproduces a permanent effect so far as the sheets ventthe escap f t gasefi Resins 0f the therare concerned, so that blisteringdoes not appear mo-setting variety which, along with suitable vein spiteof normal handling, shipping and storhicles, constitute or areingredients in the more age conditions. It is our object to provide amodern types of baking enamels comprise gentreatment of such cheapnessas not to add sigerally resins of the urea-formaldehyde types, thenifically to the cost of the sheets for enameling. phenol-aldehydetypes, the glycerol-polybasic These and other objects of our inventionacid resins, the acetylene derivatives, and the which will be set forthhereinafter or will be appoly-olefin resins. Enamels containing any ofparent to one skilled in the art upon reading these resins present ablistering problem which is these specifications, we accomplish by thatcersolved by the treatment of this invention. tain procedure andtreatment and in that cer- In the formation of our improved sheets, thetain article of which we shall now describe exferrous sheet metal isgalvanized in any suitable emplary embodiments. way by being cleanedandtreated with molten The blistering problem is by no means conzinc orshelter, though we prefer to produce our fined to galvanized sheetswhich have been pre galvanized sheets in accordance with theteachtreated to promote acceptance and adhesion of ings of Patent No.2,110,893. 7 V enamels. Blistering upon ordinary galvanized Thegalvanized sheets are then passed through sheets which have not beengiven any such treata solution containing phosphoric acid, zinc phosmentis also a problem and is equally amenable phate and an oxidizing agentsuch as sodium to the treatment which we hereinafter set forth. nitrate.This bath applies a coating of zinc A galvanized sheet which has beenBonderized" phosphate to the sheet surface. The sheets enor given anadhesion and acceptance promoting ter the bath preferably through a setof rubber treatment is, in our view, much more desirable rolls kept wetwith water, and they leave the for enameling use; and in the exemplaryembodbath through a set of rubber squeeze rolls which iment hereinaftertaken up, we shall describe our remove most of the phosphate solutionfrom the process in connection with such a sheet. sheet surface. Nextthey pass through a. cold a temperature approximately 180 F. Finallythey are passed through a set of squeeze rolls to remove the excesschromic acid solution and finally through a blast of hot air to dry thesurfaces.

The treatment above is a treatment preferred by us for giving to thegalvanized sheets an acceptance and adhesion promoting surface.

After extended research we have found that we can prevent the blisteringof baked enamels as applied either to plain galvanized sheets or sheetswhich have been Bonderized as described above by giving to the sheets aheat treatment. Curiously enough, the effect of this heat treatment ipermanent and the sheets after having once been freed of their tendencyto produce blistering in baked enamels never regain that tendency, inspite of long periods of normal handling, shipping and storage.

The heat treatment to which we refer is a specialized heat treatment ofwhich the most commercially available temperature range is from 450 to500? F., within which temperatures the treatment can be completed infrom 3 to '7 minutes or thereabouts. In investigating the effects oftemperature we havefound that heat treatments of temperatures as low as250 F. will be effective if the time duration of the treatment is longenough, say 4 to 6 hours. limit of temperature is essentially a matterdetermined by manufacturing convenience and depending upon the lengthsof time available for the heat treatment and the economics of relativelylengthy heat treatments. The upper limit of temperature is determined bythe maximum temperature which will not adversely affect the adherence ofthe galvanized coating itself to the ferrous sh'eet. Different types ofgalvanized coatings will withstand different maximum temperatures.Certain galvanized coatings applied by the lead-zinc pot process havebeen known to begin to lose their adherence at below 500 F. Regulargalvanized coatings can be heated to approximately 500 to 600 F. beforeadherence is damaged. Coatings made by the process of the patentreferred to above can be heated much higher with entire safety, and canoccasionally be heated as high as 900 F. for a short length of timebefore adherence is affected. Adherence tests for galvanized coatingsare commonly used by manufacturers and in the light of these tests madeat various temperatures the skilled Worker in the art can readilydetermine the maximum permissible. temperature for his particularproduct. Due to the shortness of the cycle (say 3 to 7 minutes) werecommend the temperature range of 450 to 550 F. for all galvanizedproducts which will withstand these temperatures. If the skilled workeris working witha product which will not withstand these temperatures, hewill be able in the light of the teachings above to select a time oftreatment consonant with a temperature low enough not toafiect theadhesion of his galvanized coating.

' Our treatment therefore is effective in producing a galvanized sheetwhich as manufactured has no tendency to produce blistering with theThus the lower 4 modern baked enamels and which, unobviously, ispermanent in this respect.

The heat treatment may be carried out in a variety of ways because thesheets may be treated either singly or in packs. Where the time cycle isshort enough to permit it, the heets may be passed singly or in groupsthrough an open heat treating furnace equipped with a conveyor (or stripmay be pulled through the furnace) or the sheets (or coils) may bestacked on a platform, covered with a box or not as desired and heatedin any suitable heat treatment apparatus capable of maintaining thedesired temperature for the desired length of time.

Modifications may be made in our invention Without departing from thespirit of it. Having thus described our invention, what we claim as newand desire to secure byLetters Patent is:

1. A process for the treatment of galvanized ferrous articles includingsteps to free them from a blistering tendency when coated withthermosetting enamels, which comprises heat treating the galvanizedsheets at a temperature andfor a time suffieient to destroy theblistering tendency, but below a temperature and time cycle at whichadherence of a galvanized coating will be adversely affected, and hencewithout altering the extent of alloy formation, said temperature beingapplied by passing the articles through a conan acceptance andadhesion-promoting treatment 7 involving the formation of an insolublezinc compound on their surfaces, and thereafter heat treating the sheetsbut below a temperature and time cycle at which adherence of agalvanized coating will be adversely affected, and hence withoutaltering the extent of alloy formation, by passing them through acontinuous annealing furnace and subjecting them to a temperature of 450to 550 F. for from 3 to 15 minutes, thetime duration of the heattreatment varying substantially inversely to the temperature thereof.

3. The process as claimed in claim 2 in which said acceptance andadhesion-promoting treatment comprises treating the galvanized sheetswith a solution of phosphoric acid, zinc phosphate and an oxidizingagent, thereafter washing said sheets, treating said sheets with achromic acid solution, and drying said sheets.

4. A process for the treatment of galvanized ferrous articles includinga treatment to free them from a blistering tendency when coated withthermosetting enamels, which comprises heat treating the galvanizedsheets at a temperature and for a time sufficient to destroy theblistering tendency but below a temperature and time cycle sufficient toimpair adhesion of a galvanized coat ing and hence without substantiallyaltering the extent of alloy formation, the temperature :of said heattreatment lying substantially within the range of 250 to 600 F. and thetime lying substantially between 3 minutes and 6 hours, the timeduration of the heat treatment varying substantially inversely to thetemperature thereof,

and thereafter coating said articles with a thermosetting enamel.

5. A process of preparing galvanized iron and steel sheets for thereception of baked enamels containing thermosetting resins and ofdestroying the tendency of said sheets to produce blistering, whichcomprises galvanizing the sheets, giving the sheets an acceptance andadhesionpromoting treatment involving the formation of an insoluble zinccompound on their surfaces and thereafter heat treating the sheets in atemperature and time cycle below that at which adherence of thegalvanized coating is impaired and hence without substantially alteringthe extent of said alloy formation, said heat treatment being attemperatures substantially within the range of 250 to 600 F. for fromsubstantially 3 minutes to substantially 6 hours, the time of heattreatment varying inversely to the temperature thereof.

6. A process of preparing galvanized iron and steel sheets for thereception of baked enamels containing thermosetting resins, and ofdestroying the tendency of said sheets to produce blistering, whichcomprises galvanizing the sheets, giving the sheets an acceptance andadhesionpromoting treatment involving the formation of an insoluble zinccompound on their surfaces and thereafter heat treating the sheets in atemperature and time cycle below that at which adherence of thegalvanized coating is impaired and hence without substantially alteringthe extent of alloy formation, said heat treatment being at atemperature of substantially 250 F. for substantially 6 hours, the saidsheets being stacked on a platform and covered with. an annealing box.

GEORGE R. HOOVER.

NOBLE E. HAYS.

